Fig. 3

a Evaluation of synthetic promoter strength. Correlation among relative mRNA expression levels (x-axis), relative protein expression levels (y-axis), and GFP fluorescence intensities (z-axis). Green circle, a strong nar promoter; yellow circle, an intermediate nar promoter; red circle, a weak nar promoter, white circle; a constitutive lac promoter, black circle; a wild-type nar promoter. b Sequence analysis of the representative synthetic nar promoters. The yellow box indicates the randomly mutated spacer sequence between the − 35 box and − 10 box. The blue boxes indicate consensus bases in the spacer region of nar promoters